arXiv subjects

Roy A. Lacey

Publications and source records attributed to Roy A. Lacey.

At least 55 records · Page 3Linked to original sources

arXiv2016

Influence of finite volume and magnetic field effects on the QCD phase diagram

The Polyakov linear sigma model (PLSM) is used to investigate the respective influence of a finite volume and a magnetic field on the quark-hadron phase boundary in the plane of baryon chemical potential ($μ_{B}$) vs. temperature ($T$) of the QCD phase diagram. The calculated results indicate sizable shifts of the quark-hadron phase boundary to lower values of $(μ_{B}~\text{and}~T)$ for increasing magnetic field strength, and an opposite shift to higher values of $(μ_{B}~\text{and}~T)$ for decreasing system volume. Such shifts could have important implications for extraction of the thermodynamic properties of the QCD phase diagram from heavy ion data.

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arXiv2016

Productions of J/psi mesons in p-Pb collisions at 5 TeV

The rapidity distributions of $J/ψ$ mesons produced in proton-lead ($p$-Pb) collisions at center-of-mass energy per nucleon pair $\sqrt{s_{NN}}=5$ TeV are studied by using a multisource thermal model and compared with the experimental data of the LHCb and ALICE Collaborations. Correspondingly, the pseudorapidity distributions are accurately obtained from the parameters extracted from the rapidity distributions. At the same time, the transverse momentum distributions in the same experiments are described by the simplest Erlang distribution which is the folding result of two exponential distributions which are contributed by the target and projectile partons respectively.

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arXiv2016

Excitation functions of parameters in Erlang distribution, Schwinger mechanism, and Tsallis statistics in RHIC BES program

Experimental results of the transverse momentum distributions of phi mesons and $Ω$ hyperons produced in gold-gold (Au-Au) collisions with different centrality intervals, measured by the STAR Collaboration at different energies (7.7, 11.5, 19.6, 27, and 39 GeV) in the beam energy scan (BES) program at the relativistic heavy ion collider (RHIC), are approximately described by the single Erlang distribution and the two-component Schwinger mechanism. Meanwhile, the STAR experimental transverse momentum distributions of negatively charged particles, produced in Au-Au collisions at RHIC BES energies, are approximately described by the two-component Erlang distribution and the single Tsallis statistics. The excitation functions of free parameters are obtained from the fit to the experimental data. A weak softest point in the string tension in $Ω$ hyperon spectra is observed at 7.7 GeV.

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arXiv2016

Searching for minimum in dependence of squared speed-of-sound on collision energy

Experimental results of the rapidity distributions of negatively charged pions produced in proton-proton ($p$-$p$) and beryllium-beryllium (Be-Be) collisions at different beam momentums, measured by the NA61/SHINE Collaboration at the super proton synchrotron (SPS), are described by a revised (three-source) Landau hydrodynamic model. The squared speed-of-sound parameter $c^2_s$ is then extracted from the width of rapidity distribution. There is a local minimum (knee point) which indicates a softest point in the equation of state (EoS) appearing at about 40$A$ GeV/$c$ (or 8.8 GeV) in $c^2_s$ excitation function [the dependence of $c^2_s$ on incident beam momentum (or center-of-mass energy)]. This knee point should be related to the searching for the onset of quark deconfinement and the critical point of quark-gluon plasma (QGP) phase transition.

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arXiv2016

Kinetic freeze-out temperature and flow velocity extracted from transverse momentum spectra of final-state light flavor particles produced in collisions at RHIC and LHC

The transverse momentum spectra of final-state light flavor particles produced in proton-proton (p-p), copper-copper (Cu-Cu), gold-gold (Au-Au), lead-lead (Pb-Pb), and proton-lead (p-Pb) collisions for different centralities at relativistic heavy ion collider (RHIC) and large hadron collider (LHC) energies are studied in the framework of a multisource thermal model. The experimental data measured by the STAR, CMS, and ALICE Collaborations are consistent with the results calculated by the multi-component Erlang distribution and Tsallis Statistics. The effective temperature and real temperature (kinetic freeze-out temperature) of interacting system at the stage of kinetic freeze-out, the mean transverse flow velocity and mean flow velocity of particles, and the relationships between them are extracted. The dependences of effective temperature and mean (transverse) momentum on rest mass, moving mass, centrality, and center-of-mass energy, and the dependences of kinetic freeze-out temperature and mean (transverse) flow velocity on centrality, center-of-mass energy, and system size are obtained.

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arXiv2016

Scaling properties of the mean multiplicity and pseudorapidity density in $e^{-}+e^{+}$, $e^{\pm}$+p, p($\bar{\mathrm{p}}$)+p, p+A and A+A(B) collisions

The pseudorapidity density (dN/deta) for p+p, p+A and A+A(B) collisions, and the mean multiplicity for ee, ep, and p+p collisions, are studied for an inclusive range of beam energies (Root_s). Characteristic scaling patterns are observed for both dN/deta and , consistent with a thermal particle production mechanism for the bulk of the soft particles produced in all of these systems. They also validate an essential role for quark participants in these collisions. The scaled values for dN/deta and are observed to factorize into contributions which depend on log(Root_s) and the number of nucleon or quark participant pairs (Npp). Quantification of these contributions give expressions which serve to systematize dN/deta and measurements spanning nearly four orders of magnitude in Root_s, and to predict their values as a function of Root_s and Npp.

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arXiv2016

Indications for a critical point in the phase diagram for hot and dense nuclear matter

Two-pion interferometry measurements are studied for a broad range of collision centralities in Au+Au (Root_s = 7.7 - 200 GeV) and Pb+Pb (Root_s = 2.76 TeV) collisions. They indicate non-monotonic excitation functions for the Gaussian emission source radii difference [(R_out)^2 - (R_side)^2], suggestive of reaction trajectories which spend a fair amount of time near a "soft point" in the equation of state (EOS) that coincides with the critical end point (CEP). A Finite-Size Scaling (FSS) analysis of these excitation functions, provides further validation tests for the CEP. It also indicates a second order phase transition at the CEP, and the values T^{cep} ~ 165 MeV and mu_B^{cep} ~ 95 MeV for its location in the (T, mu_B)-plane of the phase diagram. The static critical exponents (nu ~ 0.66 and gamma ~ 1.2) extracted via the same FSS analysis, place this CEP in the 3D Ising model universality class. A Dynamic Finite-Size Scaling analysis of the excitation functions, gives the estimate z ~ 0.87 for the dynamic critical exponent, suggesting that the associated critical expansion dynamics is dominated by the hydrodynamic sound mode.

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arXiv2015

Transverse momentum and pseudorapidity distributions of final-state particles and spatial structure pictures of interacting system in p-Pb collisions at \sqrt{s_{NN}}=5.02 TeV

The transverse momentum and pseudorapidity distributions of final-state particles produced in proton-lead (p-Pb) collisions at center-of-mass energy per nucleon pair \sqrt{s_{NN}}=5.02 TeV are studied in the framework of a multisource thermal model. Experimental results measured by the ALICE and CMS Collaborations are described by the Tsallis transverse momentum distribution and the two-cylinder pseudorapidity distribution. Based on the parameter values extracted from the transverse momentum and pseudorapidity distributions, some other quantities are extracted. Then, the structure pictures of interacting system at the stage of kinetic freeze-out in some spaces are obtained.

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arXiv2014

Observation of the critical end point in the phase diagram for hot and dense nuclear matter

Excitation functions for the Gaussian emission source radii difference ($R^2_{\text{out}} - R^2_{\text{side}}$) obtained from two-pion interferometry measurements in Au+Au ($\sqrt{s_{NN}}= 7.7 - 200$ GeV) and Pb+Pb ($\sqrt{s_{NN}}= 2.76$ TeV) collisions, are studied for a broad range of collision centralities. The observed non-monotonic excitation functions validate the finite-size scaling patterns expected for the deconfinement phase transition and the critical end point (CEP), in the temperature vs. baryon chemical potential ($T,μ_B$) plane of the nuclear matter phase diagram. A Finite-Size Scaling (FSS) analysis of these data indicate a second order phase transition with the estimates $T^{\text{cep}} \sim 165$~MeV and $μ_B^{\text{cep}} \sim 95$~MeV for the location of the critical end point. The critical exponents ($ν\sim 0.66$ and $γ\sim 1.2$) extracted via the same FSS analysis, places the CEP in the 3D Ising model universality class.

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arXiv2014

Beam energy dependence of the expansion dynamics in relativistic heavy ion collisions: Indications for the critical end point?

The flow harmonic $v_{n}$ and the emission source radii $R_{\text{out}}$, $R_{\text{side}}$ and $R_{\text{long}}$ are studied for a broad range of centrality selections and beam collision energies in Au+Au ($\sqrt{s_{NN}}= 7.7 - 200$ GeV) and Pb+Pb ($\sqrt{s_{NN}}= 2.76$ TeV) collisions at RHIC and the LHC respectively. They validate the acoustic scaling patterns expected for hydrodynamic-like expansion over the entire range of beam energies studied. The combined data sets allow estimates for the \sqsn\ dependence of the mean expansion speed $\left $, emission duration $\left<Δτ\right>$ and the viscous coefficients $\left<β''\right>$ that encode the magnitude of the specific shear viscosity $\left<η/s\right>$. The estimates indicate initial-state model independent values of $\left<η/s\right>$ which are larger for the plasma produced at 2.76 TeV (LHC) compared to that produced at 200 GeV (RHIC) ($\left<4πη/s\right>_{\text{LHC}}=2.2\pm 0.2$ and $\left<4πη/s\right>_{\text{RHIC}}=1.3\pm 0.2$). They also show a non-monotonic \sqsn\ dependence for $\left<β''\right>$, $\left $ and $\left<Δτ\right>$, with minima for $\left<β''\right>$ and $\left $, and a complimentary maximum for $\left<Δτ\right>$. These dependencies signal a significant change in reaction dynamics in a narrow span of $\sqrt{s_{NN}}$, which may be linked to reaction trajectories close to the critical end point (CEP) in the phase diagram for nuclear matter.

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arXiv2013

Acoustic scaling of anisotropic flow in shape-engineered events: implications for extraction of the specific shear viscosity of the quark gluon plasma

It is shown that the acoustic scaling patterns of anisotropic flow for different event shapes at a fixed collision centrality (shape-engineered events), provide robust constraints for the event-by-event fluctuations in the initial-state density distribution from ultrarelativistic heavy ion collisions. The empirical scaling parameters also provide a dual-path method for extracting the specific shear viscosity $(η/s)_\mathrm{QGP}$ of the quark-gluon plasma (QGP) produced in these collisions. A calibration of these scaling parameters via detailed viscous hydrodynamical model calculations, gives $(η/s)_\mathrm{QGP}$ estimates for the plasma produced in collisions of Au+Au ($\sqrt{s_{NN}}= 0.2$ TeV) and Pb+Pb ($\sqrt{s_{NN}}= 2.76$ TeV). The estimates are insensitive to the initial-state geometry models considered.

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arXiv2013

Is anisotropic flow really acoustic?

The flow harmonics for charged hadrons ($v_{n}$) and their ratios $(v_n/v_2)_{n\geq 3}$, are studied for a broad range of transverse momenta ($p_T$) and centrality ($\text{cent}$) in Pb+Pb collisions at $\sqrt{s_{NN}}= 2.76$ TeV. They indicate characteristic scaling patterns for viscous damping consistent with the dispersion relation for sound propagation in the plasma produced in the collisions. These scaling properties are not only a unique signature for anisotropic expansion modulated by the specific shear viscosity ($η/s$), they provide essential constraints for the relaxation time, a distinction between two of the leading models for initial eccentricity, as well as an extracted $\left< η/s \right>$ value which is insensitive to the initial geometry model. These constraints could be important for a more precise determination of $η/s$.

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arXiv2013

Beam energy dependence of the viscous damping of anisotropic flow

The flow harmonics $v_{2,3}$ for charged hadrons, are studied for a broad range of centrality selections and beam collision energies in Au+Au ($\sqrt{s_{NN}}= 7.7 - 200$ GeV) and Pb+Pb ($\sqrt{s_{NN}}= 2.76$ TeV) collisions. They validate the characteristic signature expected for the system size dependence of viscous damping at each collision energy studied. The extracted viscous coefficients, that encode the magnitude of the ratio of shear viscosity to entropy density $η/s$, are observed to decrease to an apparent minimum as the collision energy is increased from $\sqrt{s_{NN}}= 7.7$ to approximately 62.4 GeV; thereafter, they show a slow increase with $\sqrt{s_{NN}}$ up to 2.76 TeV. This pattern of viscous damping provides the first experimental constraint for $η/s$ in the temperature-baryon chemical potential ($T, μ_B$) plane, and could be an initial indication for decay trajectories which lie close to the critical end point in the phase diagram for nuclear matter.

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arXiv2012

Does quark number scaling breakdown in Pb+Pb collisions at Root_s = 2.76 TeV?

The anisotropy coefficient $v_2$, for unidentified and identified charged hadrons [pions ($π$), kaons ($K$) and protons ($p$)] measured in Au+Au collisions at $\sqrt{s_{NN}}= 0.20$ TeV (RHIC) and Pb+Pb collisions at $\sqrt{s_{NN}}= 2.76$ TeV (LHC), are compared for several collision centralities ($\text{cent}$) and particle transverse momenta $p_T$. In contrast to the measurements for charged hadrons, the comparisons indicate a sizable increase of $v_2(p_T)$ for $π,K$ and $p$, as well as a blueshift of proton $v_2(p_T)$, from RHIC to LHC. When this blueshift is accounted for, the LHC data [for $π$, $K$, $p$] show excellent scaling of $v_2({KE}_T)$ with the number of valence quarks ($n_q$), for a broad range of transverse kinetic energies (${KE}_T$) and collision centralities. These observations suggest a larger mean sound speed $ $ for the plasma created in LHC collisions, and significant radial flow generation after its hadronization.

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arXiv2012

Scaling patterns for azimuthal anisotropy in Pb+Pb collisions at Root_s = 2.76 TeV: Further constraints on transport coefficients

Azimuthal anisotropy measurements for charged hadrons, characterized by the second order Fourier coefficient $v_2$, are used to investigate the path length ($L$) and transverse momentum ($p_T$) dependent jet quenching patterns of the QCD medium produced in Pb+Pb collisions at $\sqrt{s_{NN}}=2.76$\,TeV. $v_2$ shows a linear decrease as $1/\sqrt{p_T}$ and a linear increase with the medium path length difference ($ΔL$) in- and out of the $Ψ_2$ event plane. These patterns compliment a prior observation of the scaling of jet quenching ($R_{\rm AA}$) measurements. Together, they suggest that radiative parton energy loss is a dominant mechanism for jet suppression, and $v_2$ stems from the difference in the parton propagation length $ΔL$.An estimate of the transport coefficient $\hat{q}$, gives a value comparable to that obtained in a prior study of the scaling properties of $R_{\rm AA}$. These results suggest that high-$p_T$ azimuthal anisotropy measurements provide strong constraints for delineating the mechanism(s) for parton energy loss, as well as for reliable extraction of $\hat{q}$.

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arXiv2012

Scaling patterns for the suppression of charged hadron yields in Pb+Pb collisions at Root_s = 2.76 TeV: Constraints on transport coefficients

Suppression measurements for charged hadrons are used to investigate the path length (L) and transverse momentum (p_T) dependent jet quenching patterns of the hot and dense QCD medium produced in Pb+Pb collisions at Root_s =2.76 TeV at the LHC. The observed scaling patterns, which are similar to those observed for Au+Au collisions at Root_s = 0.20 TeV at RHIC, show the trends predicted for jet-medium interactions dominated by radiative energy loss. They also allow a simple estimate of the transport coefficient $\hat{q}$, which suggests that the medium produced in LHC collisions is somewhat less opaque than that produced at RHIC, if the same parton-medium coupling strength is assumed. The higher temperature produced in LHC collisions could reduce the parton-medium coupling strength to give identical values for $\hat{q}$ in LHC and RHIC collisions.

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arXiv2011

Scaling of the higher-order flow harmonics: implications for initial-eccentricity models and the "viscous horizon"

The scaling properties of the flow harmonics for charged hadrons $v_{n}$ and their ratios $[ v_n/(v_2)^{n/2}]_{n\geq 3}$, are studied for a broad range of transverse momenta ($p_T$) and centrality selections in Au+Au and Pb+Pb collisions at $\sqrt{s_{NN}}=0.2 \text{and} 2.76$ TeV respectively. At relatively low $p_T$, these scaling properties are found to be compatible with the expected growth of viscous damping for sound propagation in the plasma produced in these collisions. They also provide important constraints for distinguishing between the two leading models of collision eccentricities, as well as a route to constrain the relaxation time and make estimates for the ratio of viscosity to entropy density $η/s$, and the "viscous horizon" or length-scale which characterizes the highest harmonic which survives viscous damping.

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arXiv2011

Glauber-based evaluations of the odd moments of the initial eccentricity relative to the even order participant planes

Monte Carlo simulations are used to compute the centrality dependence of the odd moments of the initial eccentricity $ε_{n+1}$, relative to the even order (n) participant planes $Ψ^*_n$ in Au+Au collisions. The results obtained for two models of the eccentricity -- the Glauber and the factorized Kharzeev-Levin-Nardi (fKLN) models -- indicate magnitudes which are essentially zero. They suggest that a possible correlation between the orientations of the the odd and even participant planes ($Ψ^*_{n+1}$ and $Ψ^*_n$ respectively), do not have a significant influence on the calculated eccentricities. An experimental verification test for correlations between the orientations of the the odd and even participant planes is also proposed.

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